A number of weather balloon launches occurred at the campus of UNC Asheville every three hours from 0600 through 1500 UTC on Monday, 6 January 2014 to document the passage of a cold front associated with an arctic air mass that resulted in the breaking of numerous minimum temperature records over the eastern U.S. overnight 6-7 January.
The first balloon launch took place at 0600 UTC 6 January 2014 (Fig. 1, top) and documented the vertical temperature and moisture profiles of the pre-frontal environment of the warm air mass (Fig. 2, top). The near-surface southeasterly wind observation at 0600 UTC is indicative of the pressure trough associated with the cold front being located to the north of Asheville, the direction also influenced by the topography of the French Broad River Valley. The freezing level at 0600 UTC is just below the 700 hPa level (711 hPa) and a warm nose (just over 9 degC) is located at ~895 hPa. The final balloon launch at 1500 UTC 6 January 2014 (Fig. 1, bottom) illustrates the substantial cooling and drying of the atmosphere below the 600 hPa level that took place during the nine hour period. Note the position of the associated surface cold front in eastern NC at 1500 UTC (Fig. 2, bottom). Air flow in the PBL at 1500 UTC was almost entirely from the northwest, but a lack of moisture at low-levels near Asheville resulted only in snow flurries and no accumulation. Operational model forecasts of the 0600 – 1500 UTC 6 January 2014 period for Asheville generally overestimated the degree of saturation in the layer between 600 hPa and the ground and, as a result, overestimated the snow accumulation when precipitation type changed over to snow by 0800 UTC.
Cold frontal passage on 6 January 2014 in western NC was associated with a strong jet stream aloft with winds exceeding 50 knots nearly extending below 1.5 km MSL at Old Fort, NC between 0400 and 0600 UTC, as indicated by the ESRL profiler (Fig. 3). The axis of the polar jet stream (Fig. 4) was well to the northwest of western NC between 0400 and 0600 UTC and so the strong winds near 1.5 km MSL during this period are associated with a momentum source distinct from the polar jet (e.g., pre-frontal low-level jet).
Figure 1. Skew T Log-p thermodynamic diagram containing observations of a rawinsonde ascent launched from the campus of UNC Asheville at (top) 0600 UTC and (bottom) 1500 UTC 6 January 2014.
Figure 2. World Prediction Center sea level pressure analyses at (top) 0600 UTC and (bottom) 1500 UTC 6 January 2014.
Figure 3. ESRL wind profiler observations at Old Fort, NC extending from 0100 UTC (right) through 1800 UTC (left) 6 January 2014. Current and archived profiler observations are available for plotting at https://madis-data.noaa.gov/cap/profiler.jsp?options=full .
Figure 4. SPC 300 hPa level observations, isotachs (shading), streamlines, and divergence (yellow contours) analysis valid 1200 UTC 6 January 2014.
To access other soundings associated with this case, please visit the Sounding-based Experiment on Mixed Precipitation Events (SEMPE) real-time data access web page at http://www.atms.unca.edu/sempe/rt_sempe1314.html. This particular case study was associated with Intensive Observation Period (IOP) #2 of SEMPE1314.